Internal combustion engines for vehicles comprising a cylinder head bearing one or more cylinders, inside which the engine stroke takes place, and which are set in communication with respective combustion chambers of said engine, are known in the prior art. Moreover, appropriate seats are provided in said cylinder head to place the combustion chamber in communication with ducts designed to carry a mixture of unburnt fuel and air into said chamber (“intake ducts”) and to carry the burnt gases away from said chamber (“exhaust ducts”).
The flows from and to each combustion chamber are controlled by specific valves that act on the aforesaid seats. In particular, each valve basically comprises a guide element, fixed within a cavity of the cylinder head of the engine and defining a through seat, and a stem, movable in opposite directions in a sliding manner within said seat and bearing at one end a shutting portion for closing the connection between the respective intake or exhaust duct and the corresponding combustion chamber.
The opposite end of the valve stem protrudes axially from the respective guide element and is suitable to receive actuating forces from a respective control mechanism, for example a camshaft.
The valve stem is axially loaded by a helical spring in the direction of closing of the connection between the respective intake or exhaust duct and the corresponding combustion chamber.
In particular, the spring is mounted coaxially around the valve and is axially interposed between a fixed surface obtained on the engine cylinder head and a plate fixed to the stem of the valve close to or at the end of said stem that cooperates with the control device.
Seal gaskets for the lubricating oil normally circulating in engines are usually mounted on the valves of the type described above. Said gaskets, in one of the most widely known forms, comprise a supporting or reinforcement member, of a substantially tubular shape and made as a one-piece metal construction, and an annular sealing element, made of elastomeric material and arranged between the supporting member and the valve
In particular, the sealing element typically comprises a first portion suitable to cooperate, via an inner radial surface thereof, with the outer radial surface of the portion of the guide element that, in use, faces said control mechanism, and a second portion designed to cooperate directly with the stem of the valve.
Gaskets of the type described above are widely used in all internal combustion engines to control of the amount of lubricating oil flowing from the distribution area to the combustion chambers. An excessive flow of lubricating oil, besides clearly resulting in excessive consumption of said oil, also causes a deterioration of engine efficiency and undermines the performance of the catalytic converter of the vehicle. On the other hand, an insufficient flow leads to increased wear and noise of the valves accompanied by the presence of local temperature peaks. These phenomena may result in premature damage to the valves due to seizing of the stem of said valves within the guide element.
With the gaskets known in the prior art, a static seal is created by the first portion of the sealing element acting on the guide element of the corresponding valve, and a dynamic seal is created by the second portion of the sealing element cooperating with the stem. In particular, the static seal must ensure a certain degree of radial compression on the guide element in order to prevent leakage of the lubricating oil towards the combustion chambers while at the same time holding said gasket in place, whereas the dynamic seal is designed to allow the minimum flow of oil necessary to lubricate the coupling between the stem and the guide element
The supporting member comprises:                a substantially cylindrical main portion;        a first annular flange, which extends radially inwards from an axial end of the main portion and is in part embedded in an annular seat of the sealing element; and        a second annular flange, which extends radially outwards from an opposite axial end of the main portion and is designed to be pushed against the aforementioned fixed surface of the engine cylinder head by the spring acting on the valve stem.        
In practice, the second annular flange of the supporting member defines an abutment surface for an axial end of the spring and receives from the latter the normal operating loads.
The second annular flange also enables the gasket to be brought into action in the desired position on the valve.
In order to reduce the weight and cost of the gaskets described above, patent application EP-B-2868875 proposed the construction of the supporting member as two separate components coupled by means of a snap fastening mechanism; the component cooperating directly with the sealing element is made of metal material, whereas the component cooperating with the spring of the valve is made of plastic material.
Though functionally valid, this solution can be further improved: it has in fact been noted that, if the gasket and the spring are not perfectly coaxial, the spring could drag along the side wall of the plastic component, which would be dangerous and could cause undesirable wear.
Moreover, in use, the spring could also project radially with respect to the second annular flange of the supporting member and so reduce the contact surface and increase the contact pressure.